1. 备份原来的yum源
$sudo cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo-backup
2.设置aliyun的yum源
$sudo wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
3.添加EPEL源
$sudo wget -P /etc/yum.repos.d/ http://mirrors.aliyun.com/repo/epel-7.repo
4.清理缓存，生成新缓存，执行yum系统更新
$sudo yum clean all
$sudo yum makecache
$sudo yum update（可选）